1. Field of the Invention
The invention relates to insulation test apparatus in which an insulated conductor under test is subjected to a high frequency voltage producing a waveform whose characteristics indicate the quality of insulation on the conductor.
2. Prior Art
High frequency voltage testing of insulated conductors is a well established technique and is widely used for checking the quality of the insulation used in dynamoelectric machine coils. The general technique involves discharging a capacitor into the coil and observing the frequency and Q of discharge voltage or current. The test also serves to identify coils which by accident may have more or fewer turns than they are supposed to have. The basic technique is described in an article by J. L. Rylander in Transactions of the AIEE, Feb. 1926, pages 459 to 465. Reference may also be made to copending application Ser. No. 146,234, filed May 24, 1971, U.S. Pat. No. 3,731,185, issued May 1, 1973, by the present inventor and assigned to the present assignee, which further discusses the background of this type of apparatus and discloses an improvement with respect to the manner in which capacitor discharge is effected.
In the apparatus originally disclosed by Rylander and as widely used the indication of discharge frequency is obtained by a wave meter. Similar apparatus has been constructed by P. Mylner and J.E. Macko, in unpublished work, using certain improvements over the original apparatus of Rylander including displaying the voltage of the capacitor on an oscilloscope as a discharge, facilitating the measurement of the discharge frequency and the circuit Q.
Further modified apparatus has been at least demonstrated by a manufacturer prior to the present invention, that includes (as schematically illustrated in FIG. 1) essentially two complete discharge circuits 10 and 20 and includes provisions for connection of a test coil 11 and 21 within each of the discharge circuits. By using a current transformer 12 on the common ground branch of the two circuits 10 and 20 and triggering them alternately, a display of the discharge current in each of the two coils alternately, but rapidly, appears on an oscilloscope 15. If one coil has faulty insulation or the wrong number of turns, it is easily detected by noticing the difference in the forms of the two traces.
The provision of duplicate discharge circuits is unattractive from a size and ecomony standpoint and the purposes of achieving a simultaneous display without such duplication led to the making of the present invention.